hich of the following is represented by $\text{'}{x}_i\text{'}$ in the center$-$of$-$gravity equation $)=({\displaystyle \underset{?}{\overset{?}{}}}{x}_i\frac{W_i}{{\displaystyle \underset{?}{\overset{?}{}}}}{W}_i?$

a$.x$ coordinate of the center of gravity

b$.x$ coordinate of location $i$

c$.x$ volume of goods moved to location i

d$.$ x volume of goods moved from location i